In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
In general terms, the use of beamforming could imply that the terminal devices will be not only operatively connected to the network node via a beam but also performs a handover between (narrow) beams instead of between network nodes of different cells. At higher frequency bands high-gain beamforming with narrow beams could be used due to more challenging radio propagation properties than at lower frequency bands. Each beam will only be optimal within a small area and the link budget outside the optimal beam will deteriorate quickly. Hence, frequent and fast beam switching is needed to maintain high performance. This is hereinafter referred to as beam management. One purpose of so-called beam management is thus for the network node to keep track of its served terminal devices with narrow beams (as used at the transmission and reception point (TRP) of the network node and/or the terminal devices) in order to increase coverage and throughput.
Due to the high penetration loss through objects and poor diffraction around object edges at higher frequency bands the link between the TRP and the terminal device will be sensitive to blocking. Blocking could occur either slowly/gradually or very suddenly, depending on the speed of movement of the terminal device, and the motion of objects in the environment. The narrower the beams, the more chance there is for sudden blocking to occur. Thus, due to rotation, movement and/or blockage of the served terminal devices the beam (at the TRP and/or terminal devices) needs to be updated dynamically in order to maintain good channel quality between the network node and the served terminal devices.
In case an operative connection between a served terminal device and the network node is lost, for example due to radio propagation channel deterioration such as blocking, a beam recovery procedure can be initiated to re-establish the beam connection. Such a beam recovery procedure could, for example, involve beam training whereby all different combinations of beams, both at the TRP and at the terminal device, are swept through. Beam training could thus be part of beam management. When there are many candidate beams such beam training could be costly in terms of time consumption and overhead signaling.
Further, in case the TRP notice a slow deterioration of the radio propagation channel of the active link the TRP could be configured to try to find a backup link to the terminal device by transmitting signals on the active link before it is lost and then transmitting control signaling that sets up a beam search procedure to find a new active link. However, in case the deterioration is too sudden there will not be enough time to setup and perform a beam search procedure, which will lead to that the active link used for control and data signals between TRP and the terminal device will be lost, thus resulting in a so-called beam link failure (BLF). If the beam link failure cannot be handled properly such that the link between the TRP and the terminal device is not restored within a certain period of time the terminal device might experience a radio link failure (RLF). RLF generally requires protocol layer 3 signaling and introduces both overhead signaling and latency.
Hence, there is still a need for improved handling of situations of radio propagation channel deterioration.